1. Field of the Invention
The present invention relates generally to a method and apparatus for finishing a workpiece. More specifically, the method and apparatus measures or monitors various operating parameters occurring during the finishing operation of a workpiece and varies different operating parameters to maintain optimum predetermined or established values.
2. Description of Related Art
Microfinishing is a unique process that removes surface defects caused by previous operations to produce a high quality finish. The process involves utilizing an abrasive fed against the workpiece under a low or constant force. As is known, the abrasive determines the rate or duration of the feed. After the abrasive removes the initial roughness and reaches the solid, base material, material removal rate is reduced and the abrasive becomes dull. This completes the geometry portion of the microfinishing process, as the abrasive no longer removes a measurable amount of the workpiece material. Continued application of the abrasive to the workpiece functions to create the required surface finish.
One of the problems associated with a microfinishing process is maintaining the effectiveness of the abrasive such that it removes the initial roughness and reaches the solid, base material of the workpiece. Depending upon the coarseness of the workpiece and the force applied on the abrasive, for example, abrasive particles located on a microfinishing film, the abrasive may fracture thus reducing the overall effectiveness of the abrasive, in this case the microfinishing film. The fracture rate of the abrasive is a function of the amount of speed and pressure put on the abrasive in relation to the surface texture of the workpiece. If the surface texture of the workpiece is coarse and too much pressure is applied to the abrasive, the abrasive will fracture which correspondingly reduces its ability to cut efficiently during the normal microfinishing cycle.
Accordingly, too much pressure causes the abrasive to fracture and too little pressure increases the overall cycle time of the microfinishing process. Typically, in order to reduce the risk of fracturing and maintaining the effectiveness of the abrasive, the microfinishing operation is based on a fixed cycle time of increased duration. In short, the abrasive is fed slowly against the workpiece at a reduced rate to correspondingly reduce or prevent fracturing of the abrasive.
Various methods for finishing a workpiece are known, see for example U.S. Pat. No. 6,782,760, that discloses a method for finishing a workpiece by controlling the feed of the processing tool based on the contact pressure. Specifically, a processing tool attached to a tool spindle advances at a pre-selected feed rate. A force measuring device measures the contact pressure applied by the processing tool on the workpiece and upon recognition of the initial cut and corresponding initial force, stops the feeding or advancing movement. Upon making initial contact, a controller fixes the rate at which the processing tool advances against the workpiece based on preset or predetermined value. If the measured value of the contact pressure or force is greater than the preset value, advancement of the feeding device used to move the processing tool varies in steps or incrementally. In addition, the initial or nominal force value may be reduced during the finishing process with the feed rate values adjusted by a controller subject to a damping function.
While controlling the feed rate to control the force applied to the processing tool can be very effective in achieving a high quality finish it typically requires starting with a low feed rate and a low force or contact pressure between the processing tool and the workpiece to prevent fracturing of the abrasive on the processing tool due to the condition of the workpiece. This process takes into account the worst-case scenario of the surface texture of the workpiece and builds into the microfinishing operation an increased cycle time to address the worst-case scenario. This equates to a fixed cycle time of somewhat longer duration than is necessary, in that a certain amount of time is used in advancing the processing tool slowly against the workpiece to reduce any undesired premature fracturing of the abrasive particles and consequently reducing their useful life.
From the above, it can be appreciated that a method and apparatus for microfinishing a workpiece that monitors and controls additional variables in the finishing process in addition to the force applied by the processing tool on the workpiece is needed. Such a method could be used to control the processing parameters and thus reduce potential failure or fracturing of the abrasive thereby increasing the useful life of the processing tool and producing a microfinishing apparatus and method that processes the workpiece in the most economical time and efficient manner.